Inductive floating CPS - Cranking

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

The pick-up ring references below relate to a number of possible components:

Flywheel

Drive plate

Pulley assembly

Timing plate

Adaptor plate

Ring gear

This test evaluates the operation of an inductive Crankshaft Position Sensor (CPS) with no ground reference, typically referred to as a floating CPS.

Note: The correct operation of the inductive floating CPS is depends on the integrity of the CPS internal coil windings, the CPS circuit, the fitment of the CPS in relation to the pick-up and the air gap between the CPS and pick-up ring.

The test procedure below assumes the conditions mentioned above are all in order and the CPS is functioning correctly. Any failures identified with the operation of the CPS while conducting these tests does not necessarily indicate a fault with the CPS itself.

The CPS will display operational characteristics that are inconsistent due to circuit faults, electromagnetic interference, mechanical failures (pick-up ring) or measurement/connection errors. The results obtained are therefore symptoms of underlying conditions and not the result of a faulty CPS. It is therefore paramount to carry out a basic inspection of all the items above before measurements are taken to prevent incorrect diagnosis of the CPS.

All numerical readings quoted in this help topic are typical and not applicable to all engine types.

Typical values
Engine cranking: Non-start

Signal ruler at zero volts should pass through the centre of the CPS waveform.

②

Indicates the missing teeth from the CPS pick-up ring, often mistaken for TDC. Note the orientation of waveform differs between Channel A and Channel B (see Diagnosis notes below).

③

Minimum amplitude of the CPS output signal voltage due to reduction in cranking speed as a result of the compression stroke(see More information below).

④

Maximum amplitude of the CPS output signal voltage due to increase in cranking speed as a result of the expansion stroke (see More information below).

⑤

Time, and rotation rulers denote one revolution of the crankshaft between the missing teeth of the pick-up ring. The Example waveforms confirm 35 teeth in one crankshaft revolution with one missing tooth denoting crankshaft position.

⑥

Indicates the cranking speed of the engine based upon the position of the time rulers ⑤. Time rulers placed at subsequent missing teeth of the pick-up ring allow PicoScope to calculate the frequency of 1 full cycle (revolution) of the crankshaft. Both the frequency and more importantly, the RPM are then displayed at point ⑥.

Diagnosis

The PCM utilises the signal on both floating CPS wires in order to determine crankshaft speed and position. Neither wire is connected to ground (hence the term floating) and therefore both carry a mirror image of the AC voltage generated by the CPS. The PCM requires the correct orientation of the signal on the relevant wire in order to identify engine speed and position.

Channel A indicates the correct orientation of a typical CPS signal and is considered as the primary or main signal input, while Channel B is referred to here as the sub-signal input. Both inputs are fundamentally identical to one another, but their orientation must be correct to the relevant CPS wire.

Always refer to the manufacturer's workshop manual for reference to PCM and CPS pin numbers to ensure the correct orientation of the CPS signal arrives at the PCM on the relevant wire.

To identify the main and sub-signals see Figure 4:

Figure 4 - Main and sub-signals

When we are measuring each floating CPS signal wire individually with reference to our test lead ground, we are unable to measure the true amplitude of the CPS signal. In most applications this is not relevant for orientation and timing/correlation inspections, but should the true amplitude be suspect, then we can apply a math channel (Channel A − Channel B) to reveal the true amplitude based upon the differential voltage of both the main and sub-signals. See Figure 5.

To display the built-in Maths Channel A-B to the captured CPS signals, select: Tools > Math Channels > enable the box adjacent to A-B and select OK. A third math channel will appear displaying the true amplitude of the floating CPS.

Figure 5 - A-B Maths Channel and True amplitude

The PCM calculates the differential voltage derived from the main and sub-signals present on both wires of the CPS. Using the Math Channel (A-B) above, allows the scope to reveal the true amplitude of the CPS as utilised by the PCM to identify engine speed and position.

More information

The CPS is one of the fundamental components of the modern engine management system. While beautifully simple in construction, the correct operation of the CPS is paramount to an efficient running engine. To fully appreciate the role played by the inductive CPS we must understand the operational principle behind the AC voltage generated by such a critical component. Once we understand how this component functions, we can evaluate the cause of any errors within our waveform.

Operating principle

The inductive CPS consists of two essential components: a wire coil wound about a permanent magnet.

Figure 6 - CPS Operating principle

Each end of the wire coil terminates at the PCM where the generated AC output signal voltage (present on both wires) is utilised by the PCM to indicate crankshaft speed and position (see Figure 6).

Generation of AC voltage

The permanent magnet contained with the CPS naturally produces a magnetic field about the coil windings. Should a metallic object be introduced into this magnetic field (in the form of our pick-up ring) the intensity of the magnetic field will vary and either increase or decrease depending on the speed and direction of travel of the pick-up ring (see Figure 7). The variation in the magnetic field has the desired effect of inducing an AC voltage into the coil winding that can be utilised by the PCM to derive engine speed and position.

N.B The variation in the magnetic field is solely responsible for inducing an AC voltage into the coil windings. Should the pick-up ring come to a halt, no voltage is generated regardless of the position of the pick-up ring in relation to the CPS.

The CPS waveform will include a dropout in the signal where the pick-up ring has a deliberate gap (missing tooth or teeth) in the even spacing between the teeth (see Figure 8 and Figure 3).

The PCM uses this dropout in the signal to identify the position of the crankshaft which may or may not refer to TDC. Manufacturers use the missing teeth from the pick-up ring to indicate a variety of crankshaft positions. Such as: Pistons in line, (engine safe position) TDC, the No. of degrees before TDC, or they may choose a combination of missing teeth at 90 degree intervals. For accurate evaluation of the crankshaft position reference signal refer to the relevant workshop manual.

Crankshaft speed calculation is based on the frequency of the AC output signal from the CPS. As the speed of the crankshaft increases the frequency of the CPS output signal increases in direct proportion. The amplitude of the signal also increases with engine speed, with amplitudes above 20 V AC at higher engine speeds (see Figure 9).

Figure 9

The rise and fall in the frequency/amplitude can be seen in both Figure 2 and Figure 3 where the load placed on the crankshaft during the compression stroke results in a momentary reduction in crankshaft speed ③. Here we can see a decrease in frequency and a fall in the amplitude in direct contrast to ④ where the crankshaft momentarily increases in speed just after TDC compression (expansion stroke) resulting in an increase in frequency and a rise in amplitude.

The rise and fall in both frequency and amplitude can be used as an indication on the condition of engine compression. Assuming all engine compressions are equal, we should obtain a uniform frequency and amplitude in the CPS signal with a constant cranking speed.

When the complaint concerns poor starting or prolonged cranking, it is vital to consider the cranking speed input to the PCM.

Cranking speed can be calculated from the CPS signal by placing the time rulers at each consecutive dropout points in the captured signal (missing tooth/teeth) see Figure 2 ②.

Most pick-up rings contain 36 or 60 teeth with 1 or 2 missing teeth at a single point about their circumference. Placing the time rulers directly at the dropout points will indicate 1 x crankshaft revolution. In the Example Waveforms above we have 35 teeth between the time rulers, so indicting a pick-up ring with 35 teeth about the circumference with 1 missing tooth for engine position reference (36 teeth − 1 tooth = 1 x crankshaft revolution).

Placing a time ruler at consecutive dropout points within the waveform allows PicoScope to calculate the cranking speed based upon the frequency of the crankshaft (cycles per second).

In Figure 2 each time ruler is positioned at the point of the missing teeth ② (one crankshaft revolution) where the frequency between the time rulers is calculated at 3.954 Hz in the frequency legend ⑥.

In order to convert Frequency to RPM we simply multiply 3.954 Hz by 60 to obtain 237.26 RPM. Both the frequency and RPM value of the crankshaft are displayed in the frequency legend ⑥.

Troubleshooting (Circuit tests)

Crankshaft position sensor installed and disconnected from the vehicle harness

The resistance value of the wire coil wound around the permanent magnet provides an indication of the integrity of the CPS and is essential to the correct output voltage/signal. Incorrect resistance values or a short circuit to vehicle ground will result in output signal failure.

In order to test the resistance value of the CPS, disconnect the sensor from the vehicle harness and connect an ohmmeter as illustrated in Figure 10 below:

Figure 10

Figure 11

While the CPS resistance may be in specification, a check for a short circuit to ground can also be performed very quickly at the same time as the resistance check. Figure 11 indicates how the ohmmeter is connected to confirm no short circuit to vehicle ground is present. A value greater than 10 kilohms is sufficient to confirm no short circuit exists (an infinite or open circuit resistance value is ideal).

Crankshaft position sensor installed and connected to the vehicle harness

Depending on manufacturer a bias voltage may be present on either wire of the CPS. The bias voltage is provided by the PCM and is used as an aid to diagnose CPS circuit errors and noise reduction (interference from neighbouring electrical components).

In order to measure the bias voltage, connect PicoScope exactly as mentioned above in How to perform the test (Figure 1) and follow Steps 1 through 7, followed by:

Select DC coupling for both Channel A and Channel B (see Figure 12).

Run your scope software by pressing either the space bar on your keyboard or the Go button in PicoScope.

Switch on the ignition (do not crank or start engine).

Press the Stop button in PicoScope to halt the capture to enable waveform analysis.

Figure 12

Troubleshooting (Mechanical tests)

If the waveform indicates a fault, make a physical inspection of the CPS. The list below highlights some key areas for inspection.

CPS fitment to housing / mounting. The sensor must sit correctly and securely.

Inspect CPS tip for damage / foreign object.

Inspect pick-up ring for damage / foreign object.

Inspect air gap between CPS and pick-up ring.

Inspect run-out of pick-up ring.

Inspect end float at the pick-up ring.

Inspect CPS for water / coolant ingress and corrosion.

Inspect CPS and PCM connector pin number alignment as per vehicle specification.

Disclaimer
This help topic is subject to changes without notification. The information within is carefully checked and considered
to be correct. This information is an example of our investigations and findings and is not a definitive procedure.
Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test
settings.